The field of this disclosure relates to thin film deposition systems and methods for coating flexible substrates.
Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy (“ALE”), is a thin film deposition process that is known for use in manufacturing electroluminescent (EL) display panels, in semiconductor integrated circuit manufacturing, and for other purposes. See U.S. Pat. No. 4,058,430 of Suntola et al., and U.S. Patent Application Publication Nos. US 2004/0208994 A1 of Härkönen et al., US 2004/0124131 A1 of Aitchison et al., and US 2005/0011555 A1 of Maula et al., the specifications of which are all incorporated herein by reference. ALD offers several benefits over other thin film deposition methods, such as physical vapor deposition (“PVD”) (e.g., evaporation or sputtering) and chemical vapor deposition (“CVD”), as described in Atomic Layer Epitaxy (T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990), incorporated herein by reference.
In contrast to CVD, in which the flows of precursors are static (i.e., flow rates are steady during processing) and the substrate is exposed to multiple precursors simultaneously present in the reaction chamber, the precursor flows in ALD processing are dynamic and sequential, so that the substrate is exposed to only one precursor at a time. Successful ALD growth has conventionally required the sequential introduction of two or more different precursor vapors into a reaction space around a substrate. ALD is usually performed at elevated temperatures and low pressures. For example, the reaction space may be heated to between 200° C. and 600° C. and operated at a pressure of between 0.1 mbar and 50 mbar. In a typical ALD reactor, the reaction space is bounded by a reaction chamber sized to accommodate one or more substrates. One or more precursor material delivery systems (also known as “precursor sources”) are typically provided for feeding precursor materials into the reaction chamber.
After the substrates are loaded into the reaction chamber and heated to a desired processing temperature, a first precursor vapor is directed over the substrates. Some of the precursor vapor chemisorbs or adsorbs on the surface of the substrates to make a monolayer film. In pure ALD, the molecules of precursor vapor will not attach to other like molecules and the process is therefore self-limiting. Next, the reaction space is purged to remove excess of the first vapor and any volatile reaction products. Purging is typically accomplished by flushing the reaction space with an inert purge gas that is non-reactive with the first precursor. After purging, a second precursor vapor is introduced. Molecules of the second precursor vapor chemisorb or otherwise react with the chemisorbed or adsorbed first precursor molecules to form a thin film product of the first and second precursors. To complete the ALD cycle, the reaction space is again purged with an inert purge gas to remove any excess of the second vapor as well as any volatile reaction products. The steps of first precursor pulse, purge, second precursor pulse, and purge are typically repeated hundreds or thousands of times until the desired thickness of the film is achieved.
The required temperatures, pressures, and reaction chamber conditions have conventionally limited the ALD technique to deposition on substrates of relatively small size. For example, known uses of ALD include EL display panels and semiconductor wafers.